To obtain an L-ascorbic acid derivative which is a monomer useful for producing an oxygen-removing polymer and a polymeric L-ascorbic acid derivative formed by bonding an L-ascorbic acid unit to a polymer chain, and to provide a simple processes for producing them without using an enzyme or an acid chloride.

This L-ascorbic acid derivative having a structure expressed by the formula (R is a residue selected from maleic acid, itaconic acid and half esters of the acids) is produced by half-esterifying maleic anhydride, itaconic anhydride or a polymer which contains a monomer unit of either of them with L-ascorbic acid in the presence of a polymerization inhibitor and a basic condensing agent.